From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in
 dumps
Date: Sat, 11 Nov 2017 23:09:24 +0900 (KST)
Message-ID: <20171111.230924.412544731735070222.davem@davemloft.net>
References: <20171109014218.20562-1-Jason@zx2c4.com>
        <20171109040444.1728-1-Jason@zx2c4.com>
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Cc: johannes@sipsolutions.net, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
To: Jason@zx2c4.com
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From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Thu,  9 Nov 2017 13:04:44 +0900

> @@ -2195,13 +2197,15 @@ static int netlink_dump(struct sock *sk)
>  		return 0;
>  	}
>  
> -	nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
> -	if (!nlh)
> +	nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
> +			       sizeof(nlk->dump_done_errno), NLM_F_MULTI);
> +	if (WARN_ON(!nlh))
>  		goto errout_skb;

If you're handling this by forcing another read() to procude the
NLMSG_DONE, then you have no reason to WARN_ON() here.

In fact you are adding a WARN_ON() which is trivially triggerable by
any user.